In recent years, research and development of a white light-emitting device using a blue light-emitting element as a light source has been actively carried out. In particular, since a white light-emitting device using a blue light-emitting element not only has a long service life and smaller power consumption than an incandescent lamp and a fluorescent lamp, but also uses no hazardous substance such as mercury, practical use of lighting equipment using a white light-emitting device are presently well underway.
A method most frequently used to obtain white light using blue color of a blue light-emitting element as a light source is a method of obtaining a pseudo-white color by mixing blue with yellow, which is the complementary color of blue.
For example, in a typical white light-emitting device, a blue light-emitting element is sealed with a transparent resin containing a yellow light-emitting phosphor (for example, Y3Al5O12 (YAG) phosphor). The blue light-emitting element emits blue light (wavelength: from 450 to 460 nm) and parts of the blue light excite a YAG phosphor, from which yellow light is emitted.
However, when a blue light-emitting element is sealed with a light-transmitting resin such as an epoxy resin, it is known that the light emitted from a light-emitting element or the light from outside reacts with the epoxy resin leading to deterioration such as yellowing. Also, when a high current is used to obtain high brightness in a white light-emitting device, the element itself generates heat to cause deterioration of a sealing resin. Further, the sealing resin absorbs moisture and the like potentially decreasing the luminous efficacy.
Therefore, the present inventors have proposed a white light-emitting device provided with a blue light-emitting element and a ceramic composite for light conversion formed of a solidified body formed by continuously and three-dimensionally entangling with each other the plurality of crystalline phases of oxides containing an Y3Al5O12 phase (hereinafter denoted as YAG:Ce phase) containing fluorescence emitting Ce and an Al2O3 phase. A ceramic composite for light conversion can generate stable yellow fluorescence with homogenous distribution because of uniform distribution of the phosphor phase, and has excellent durability because of ceramic, thereby solving the problems generated when sealed with an epoxy resin and the like, and a white light-emitting device with high reliability can be provided.
The structure of a white light-emitting device using the ceramic composite for light conversion includes, for example, a blue light-emitting element to be flip-chip mounded, a circuit board on which a wiring pattern is formed to supply power to the blue light-emitting element, and a ceramic composite for light conversion directly bonded to the blue light-emitting element.
As described in Patent Literature 1, such a substrate for a light-emitting diode was previously proposed that a single-crystal layer enabling the formation of a light-emitting diode element and a ceramic composite layer for light conversion formed of a solidified body formed by continuously and three-dimensionally entangling with each other the crystalline phases of at least two kinds or more of oxides selected from a single metal oxide and complex metal oxides, are stacked. In the document, a method of direct joining at high temperature or a method of interposing a very small amount of a low melting material as a joining layer were disclosed in order to stack the single-crystal layer and the ceramic composite layer for light conversion, but when the joining surface between the single-crystal layer and the ceramic composite layer for light conversion is flat, not only the above method but also a surface activated bonding method can be used for directing bonding between the ceramic composite layer for light conversion and the single-crystal layer, so that flatness of the bonding surface is very important.